An RFID system essentially comprises an RFID read/write device (reader) and electronic tags. The latter are able to operate passively, i.e. without a battery, but are reliant on the continual presence of a carrier signal transmitted by the read device, this carrier signal also being called a power carrier. RFID systems with read ranges of several meters use UHF or microwave frequencies. The RFID read/write device itself comprises a transmitter with a baseband assembly and a radio-frequency assembly (RF assembly) and also a receiver consisting of a radio-frequency part (RF part) and a baseband part.
To achieve reading distances in the range of a few meters, a transmission power of approximately 1 watt (30 dBm) needs to be generated and radiated in the case of passive electronic tags. On the other hand, receivers in the RFID read/write devices need to detect the low level of the response signal which is reflected by the tags. As the density of RFID read/write devices within a given 3D volume increases there is the risk of system interference by the individual RFID signals, which, in comparison with the tag response signals, often arrive at the receiver with higher signal level than the response signal.
A numerical example for a transmission power of +30 dBm generated at a distance of 10 m from an RFID read/write device operated at 868 MHz results in a jamming noise signal level of −20 dBm at the receiver input. However, the useful signal from the tag at a distance of approximately 4 m is just −70 dBm in the UHF range. The digital representation of the obviously weak useful signal downstream of an A/D converter in the receiver is thus characterized by a low resolution. If indeed a plurality of RFID signals are received, communication with tags is barely possible any longer, since besides the signals from the RFID read/write devices there are also a large number of intermodulation products further occupying the useful frequency band. Although it is proposed, based on national radio regulations, that an RFID read/write device performs a Listen Before Talk (LBT) operation before the start of transmission, in order to check whether the frequency channel is not in use, this results in a high level of inefficiency in applications with a large number of uncoordinated RFID read/write devices. Individual RFID read/write devices can be intermittently shaded by moving objects and still cause interference.
Other RFID read/write devices in turn do not cause interference when they are transmitting simultaneously because their antennas are currently radiating in a different direction. Interference by other RFID read/write devices can therefore be expected at any time. Methods which provide the individual RFID read/write devices with synchronous timeslots are inefficient because there are often no tags in front of an RFID read/write device and in that case the timeslot passes unused. There are often not enough frequency bands available for separating the RFID signals in the frequency range. At best, the frequency hopping method which the national radio regulations may permit for RFID can alleviate the situation somewhat by virtue of a plurality of RFID read/write devices sharing at least some frequencies, and the number of collisions being lower.
The regulating authorities for RFID in the UHF and microwave ranges propose introducing a LBT phase ahead of an interrogation cycle, said phase involving the transmitter in an RFID read/write device being off and the received signal being analyzed over a time interval T0. This received signal contains the interrogation signals from other RFID read/write devices, and it is possible to decide whether or not the transmitter is allowed to be turned on. If the RFID read/write devices have suitable means which allow them to be identified then a plurality of RFID read/write devices are independently able to consult with one another in a cooperative sense in order to avoid mutual interference. Such consultation may be made possible by adding a Bluetooth (BT) or wireless LAN (WLAN) function to the RFID read/write device, for example.
Combined RFID read/write devices which contain RFID and BT, as described in US2002/0126013 and CA 2405894, can obviously also be used in this way for cooperation between read devices. The drawback of BT or WLAN based consultation is the higher level of hardware complexity or additional occupancy of the frequency band (e.g. RFID in the 2.4 GHz ISM band). An improved variant as presented in WO 2004/004196 could be achieved by reconfiguring a Software Defined Radio baseband part (SDR) contained in an RFID read/write device to produce a BT or WLAN function. The drawback of this method is that it is firstly suitable only for frequency bands which permit RFID and BT or WLAN emission (currently only ISM 2.4 GHz) and secondly that reconfiguring the SDR loses valuable operating time for the RFID read/write process while the SDR is performing BT or WLAN functions, for example.
WO 2004/015614 proposes synchronizing all the RFID carrier signals from multiple RFID read/write devices in order to prevent beat frequency signals (AC) in direct conversion receivers which would not be easy to filter out. The interfering AC components are produced by the slightly differing crystal frequencies in each RFID read/write device. This synchronization is complex, however, because it requires a reference transmitter or wiring for the RFID read/write devices. Besides the aforementioned BT or WLAN functions, wired networking by Ethernet or an RS232 interface of adjacent RFID read/write devices may be mentioned, or similar means in which a master undertakes time-based coordination of all read operations.
A conventional RFID read/write device is shown in FIG. 1. The present technology is based on what is known as Software Defined Radio (SDR). An RFID read/write device 10 comprises a software defined baseband part (SDR) 11 and an RF part 12. In SDR based transmission/reception installations of this kind, the complex-value signals are conditioned or processed purely arithmetically in a signal processor 13 such that they now need only be shifted into the radio frequency band by means of linear converters (up-converter and down-converter). A TX converter 17 in the transmitter is fed with a complex baseband signal (in phase and quadrature signals) which is output by the signal processor 13 via a double digital/analog (D/A) converter 15. The output signal is forwarded to the transmission antenna 19. From the reception antenna 18, received signals are converted into a complex baseband signal (in phase and quadrature signals) by means of an RX converter 16 and are forwarded to a double analog/digital (A/D) converter 14 and accepted by the signal processor 13. A known reception architecture uses what is known as a Direct Conversion Stage (DCS) for the RX converter 16 in order to get from the radio frequency (RF) into baseband. For the purpose of signaling between RFID read/write devices, a BT or WLAN baseband stage 21 can be incorporated into the device, which baseband stage can in the best case also use the converters 16, 17 and antennas 18, 19.